I play the guitar and I am not using any midi device. My song has a chorus with e.g. C/ F/ Dm /G7.
How can I generate harmonies on these chords?
This is a perfect chord change pattern for Scale harmony IF you're only using a parallel 3rd harmony! Preset 12 has a default scale of C Major. Playing the above chords while singing into this preset will be OK until you hit the G7.
There are less melody notes that you can sing that will sound right with this chord. By turning off the two voices in this preset that sing the 5th harmony (in the Voices and Effects group the rightmost 2 buttons turned off), you'll be able to jam out on pretty much any melody you like over these chord change.
If you still want the 5th harmony added to your voice, you can re-enable the other two voices but you may have to modify the melody you sing over the G7 chord.